Refrigeration The safe storage of food necessitates that it is maintained at low temperatures which requires the process of refrigeration. For bulk foods one large industrial refrigeration plant will serve separate cold rooms for the storage of meat, fruit and vegetables, dairy products, etc. Smaller domestic sized refrigerators are used to meet the daily catering needs in the galley, pantries, duty messrooms and in cabins. The refrigeration process is also utilised in deep-freezers, water-chillers and air-conditioning plant. Large scale cargo space refrigeration is also necessary for the transportation of foods and certain liquid chemicals and gases. Whatever the size or role of the ship's refrigerators, the basic principle is common:they all have an evaporator (cooling unit), a refrigerant and a condenser.
The refrigerant is generally Freon-12 (CC1 2 F 2 ) or Freon-22, but ammonia is also used in large systems. Freon refrigerants in general use are colourless and almost odourless, while also being non-toxic, non-corrosive and non flammable. However, when exposed to an open flame, a highly toxic phosgene gas is produced. Additional components to the basic refrigerant cycle may include filter-driers, heat exchangers, accumulators and pre-coolers. Also required are the operating and protective controls such as thermostats, relays, defrost controls and overcurrent trips.
Above the domestic sized refrigerator, the compressor motor will invariably be a 3-phase type driving a reciprocating compressor. The domestic version will usually be a single- phase motor driving a rotary compressor. Each cold room is fitted with a thermostat which operates a solenoid valve between set temperature limits. The quantity of refrigerant flowing in the system is regulated by the expansion valve. This valve is controlled by a liquid phial connected by a capillary tube attached to the vapour return pipe at the outlet of the evaporator. When the room temperature falls to the pre-set level, the thermostat de-energises the solenoid valve to stop circulation of the refrigerant. The resulting pressure drop in the compressor suction line will operate a low-pressure cut-out valve and stop the compressor. The rooms or compartments are cooled by natural air circulation through the evaporator coils or by forced-air from a fan blowing across a bank of cooling tubes.
In a domestic refrigerator the cooling effort is controlled by using a control thermostat to switch the compressor on or off. The hermetically sealed compressor motor is the split-phase type having two separate windings start and run. The motor is accelerated by connecting both start and run phase windings to.the supply. When the motor reaches about 80% of its rated speed, the start winding is tripped out of circuit. For compressor drives, this switch is usually in the form of a current-operated relay which is fitted adjacent to the compressor. Additionally, there may be condenser and evaporator fans which are driven by single-phase shaded-pole type motors.
Air Conditioning Air conditioning is a process which heats, cools, cleans and circulates air together with the control of its moisture content. The air must be delivered to a room with a definite temperature and specified relative humidity. For summer duty, the usual method is to cool the incoming air to a temperature below the dew point to allow condensation to occur until the mixture has the desired specific humidity then heating the air to the required delivery temperature and relative humidity. In winter, the incoming air may have to be heated and have water added to achieve the correct inlet conditions. In most plants the bulk of the mixture is re-circulating air with fresh air intake forming about one third of the total required. The amount of make-up air is a statutory requirement which is typically between 17 m 3 /hr and 28 m 3 /hr.
The electrical aspects of accommodation air conditioning (A/C) comprises the power equipment of motors and starters for the compressor(s), fans and sea-water cooling pumps. Associated control equipment will include electric solenoid valves, high and low- pressure and temperature switches together with safety cut-outs for overcurrent, loss of refrigerant, low compressor oil pressure, etc. The usual air-conditioning system used for the accommodation spaces of cargo ships is the central single-duct type. In its simplest form a single compressor serves the whole accommodation.
The compressor is generally a multi- cylinder reciprocating type with a power rating in the range of 50-200 kW, although rotary-vane or screw-action compressors may also be encountered. Large passenger vessels may have a total power requirement of more than 5 MW for the AC compressor drives to maintain air delivery to the hotel and staff accommodation areas. Capacity control of the reciprocating compressor is by automatic unloading of cylinders by valve control using servo oil pressure. The compressor, air fan and sea water pump are driven by simple fixed speed, 3-phase a.c. induction motors each with its own starter and supplied from a distribution board fitted in the air-conditioning plant room.
Routine electrical maintenance and fault finding on the motors and starters will involve cleaning, checking of connections, IR (megger)/continuity tests and running tests. Inspection of connections and correct operation of any electric heaters must also be performed. Such heaters may be used for heating the compressor crankcase oil and for separating the refrigerant (Fréon R12 or R22) from the oil in an oil reservoir. Regular inspection and testing of control and safety thermostats and pressurestats should be carried out in accordance with the manufacturer's instructions. In particular the compressor's low oil pressure alarm and trip circuit should be tested periodically for correct operation.
Galley and Laundry The electrical power in a galley is largely absorbed in producing heat. Ovens, deep fryer pans, water boilers and the hotplates on the galley range all employ resistive heating elements which are usually controlled by bimetallic thermostats. Other miscellaneous electrical galley equipment may include oven air circulating and range exhaust fans, meat slicers, food mixers and grinders, dishwashers, potato peelers and garbage disposal units. Most of this equipment will utilise small electric motors together with the necessary control switches, safety interlocks and indicator lamps. Because of the large power requirement for food preparation and cooking, the major galley items are supplied from the 3-phase a.c. 440 V system. Smaller galleys may be supplied from the low- voltage 220 V a.c. system. The electrical equipment has to work safely in the usual galley atmosphere of high humidity and high temperature.
Catering staff have been known to wash down ovens with an enthusiasm that demonstrates a scant regard for Ohm's Law! All in all, the galley electrics work in a tough area so be prepared for faults caused by the environmental hazards of grease, dust and dampness. Microwave ovens provide rapid defrosting and cooking of foods. The microwaves are produced by a special valve called a magnetron operating at around 4000 V with a frequency of 2450 MHz. Specialised knowledge is required for the repair of this type of oven and internal fault finding is not recommended without the manufacturer's guidance.
Inspection and maintenance of galley equipment is most important. The main objective is to keep the electrical parts clean and free of water, oil, dust and grease. Pay particular attention to all connection points in high current heating circuits where loose connections cause overheating and future problems. For operator safety, all enclosure metalwork must be earthed and regular checks of earthing straps must be given priority. Insulation resistance (IR) tests on heating elements, when cold, may reveal surprisingly low values (10-100 kΩ) even with new elements. This is because the element insulation (magnesium-oxide powder) is somewhat hygroscopic (absorbs moisture). The insulation resistance value of a healthy heating element should rise rapidly after being operated for a few minutes. Obviously, if the IR value of an element remains low when hot it is defective and must be replaced.
Laundry Washing machines, spin dryers and tumble dryers utilise heat and mechanical rotation during their laundry processes. The sequence of events is controlled by timers which are often simple electric timer motors driving cam- operated switches. Alternatively, electronic timers with relay switching or solid state electronic switching using thyristors or triacs may be employed. Small washing machines operating on a single- phase supply have motors which are usually the split-phase type of the capacitor-start, capacitor- run variety.
Larger washing machines operate from the 3-phase a.c. power supply with a 3-phase induction motor drive. Control items in a washing machine include water level switches, temperature switches (bi-metallic) and solenoid valves in the inlet and outlet water lines. Lid and door switches interrupt the main power supply if operated after the washing sequence has begun. Spin dryers have a safety door interlock that prevents it being opened while the drum is still revolving. Tumble dryers often only have one motor with a double- ended shaft for drum and blower fan drives. Lint and fluff collects on the motor and wiring which causes no trouble while it remains dry and in small quantities. Periodic removal of the fluff will help prevent faults arising where dampness may combine with the fluff to cause conductive tracking between live conductors and to earth. Small single-phase motors are sometimes protected by a thermal cut-out attached to the stator end windings.
Cathodic Protection The outer surface of a ship's hull is subjected to electro-chemical attack by corrosive currents that flow between areas of the hull which are at slightly different electric potentials. Dissimilar metals, variations in structural and chemical uniformity in hull plates and welding, differences in paint thickness and quality, water temperature, salinity and aeration all combine to cause areas of the hull to become either anodic (positive) or cathodic (negative).
In the hull, electrons flow from anode to cathode leaving positively charged iron ions at the anodic area. At the cathode the effect of the arrival of electrons is to produce negatively charged hydroxyl ions (OH) by electrolysis of the sea water. These negative ions flow through the sea to the anodic area where they combine with the positive iron ions to form ferrous hydroxide Fe(OH) 2. This ferrous hydroxide is further oxidised by dissolved oxygen to form ferric hydroxide Fe(OH)3 which is rust. Thus the anodic area is gradually corroded away while no corrosion takes place at the cathodic area. This naturally corrosive action can be overcome if the complete hull is made cathodic, i.e. electrons are allowed to arrive at the hull surface and produce negative hydroxyl ions but no electrons leave the hull to produce positive iron ions. This is achieved by fitting insulated lead or platinised titanium anodes to the hull and applying a positive d.c. potential to them with respect to the hull.
Cathodic protection systems fitted in ships consist of a number of anodes (lead or platinised titanium) fitted to the hull at selected places below the waterline, and control equipment which automatically regulates the anode current to the required value. Direct current is supplied to the anodes, after transformation and rectification, from the ship's 440 V 60 Hz 3- phase a.c. distribution system. The control equipment comprises reference electrodes, an amplifier assembly and one or more transformer rectifier units.
The control equipment automatically monitors the size of anode current required which will vary with the ship's speed, water temperature and salinity, condition of paint work etc. Typical anode current densities range from 10 mA/m 2 to 40 mA/m 2 for the protection of painted surfaces and 100 to 150 mA/m 2 for bare steel surfaces. The total impressed current for a hull in good condition may be as low as 20 A. Maximum controller outputs may be up to about 600 A at 8 V. Cathodic protection does not appear to deter molluscular growth on the ships hull, so a top coat of anti-foul (poisonous) paint is still necessary. Monitoring facilities in the cathodic protection control cabinet may provide measurements of: 1.Reference electrode voltage (hull potential) 2.Amplifier output voltage 3.Total anode current 4.Individual anode current
Measurements should be regularly logged together with the ship operating conditions, e.g. location, draught, water temperature, etc. Changes in underwater hull area, speed, water temperature/ salinity and paint condition will all cause the anode currents to vary. The hull potential should, however, remain constant in a properly regulated system.
Battery Supplies A properly maintained storage battery will instantly supply electric power when required. This feature makes a battery the key element in the provision of essential and emergency power supplies on board ships. Essential routine power supplies, e.g. for radio equipment, telephone exchange, fire detection, general alarm circuits etc., are often supplied from two sets of batteries worked on a regular charge/ discharge cycle.
Emergency battery supplies, e.g. for emergency generator start-up and emergency lighting, are used in a standby role to give power when the main supply fails. Ships' batteries are usually rated at a nominal voltage of 24 V d.c. In some cases a battery system of 110 V or 220 V d.c. may be used where a large amount of emergency lighting and power is vital or where a battery is the only source of emergency power.
The two main types of rechargeable battery cell are: Lead-acid Alkaline
Battery installations for both types of battery are similar in that the battery room should be well ventilated, clean and dry. Both types generate hydrogen gas during charging so smoking and naked flames must be prohibited in the vicinity of the batteries. Steelwork and decks adjacent to lead-acid batteries should be covered with acid-resisting paint and alkali resisting paint used near Ni-cad cells. Acid cells must never be placed near alkaline cells otherwise rapid electrolytic corrosion to metalwork and damage to both batteries is certain. For similar reasons, never use lead-acid battery maintenance gear (e.g. hydrometer, topping up bottles, etc.) on an alkaline installation or vice-versa.
Battery maintenance includes keeping the cell tops clean and dry, checking the tightness of terminal nuts and applying a smear of petroleum jelly to such connections to prevent corrosion. Be most careful when handling the battery electrolyte (e.g. when using a hydrometer to check its specific gravity). Use protective rubber gloves and eye goggles when handling electrolyte. Insulated spanners should be available for use on cell connections to prevent accidental short-circuiting of battery terminals. Such a short-circuit across the terminals of just one cell of a battery will cause a blinding flash with the probability of the cell being seriously damaged.